1. Field of the Invention
This invention relates to printed circuit boards (PCB's). More particularly, it relates to printed circuit boards having electronic components the leads of which are soldered to the PCB using a wave soldering process.
2. Description of the Problem and the Related Art
Printed circuit boards are most commonly laminated of glass-filled epoxy insulating layers and layers of etched copper conductors. The simplest interconnection structure consists of a single layer of insulation made of a glass-filled epoxy and a single layer of copper conductors. More complex structures consist of numerous layers of conductors and insulators which are divided into power distribution layers, signal layers, and ground plane layers. Signal layers are grouped in pairs which carry electrical information in orthogonal directions (x, y, pairs) and are located adjacent to ground layers. This provides a uniform impedance for the electrical signals and prevents electrical coupling of signals among adjacent conductors. A typical, multilayer printed circuit board with signal layers and ground and power layers is shown in cross section in FIG. 9.
In integrated structures such as those associated with semiconductor circuits (also magnetic bubble circuits and Josephson circuitry) commonly used in digital computers, the first level of packaging consists of a number of switching circuits fabricated on a single part called a chip. The chip is mounted in a structure called a module, which provides environmental protection for the chip and permits electrical interconnection to be made to the next level of package. Modules are normally provided in standard configurations to allow for the design of general-purpose manufacturing equipment and higher-level packages.
Signal wires (traces) on the printed circuit board carry signals between modules and between modules and input/output connections to the circuit board. On computer circuit boards the terminals that carry the logic signals to the modules and to the input/output terminals are called logic service terminals (LST's), those that supply power to the various modules are called power service terminals (PST's), and interconnections between wiring layers in the board are called vias or via service terminals (VST's). FIG. 9 shows the cross section of a typical multilayer circuit board having VST, LST, and PST connections between solder pads on the exterior of the circuit board and traces in internal layers of the board.
Vias may be "blind" holes which are open to one surface of the circuit board but do not extend to the opposite surface; they may be "buried" holes which connect only internal layers of the board and do not extend to either surface of the printed circuit board; or they may be plated through-holes which extend the full thickness of the board and open to solder pads on both surfaces of the board. Like any other relatively small, plated through-hole, a via hole which extends the full thickness of a PCB will most commonly fill with solder due to capillary action when the PCB is subjected to wave soldering.
If two clean metal surfaces are held together and dipped into molten solder, the solder will wet the metal and climb up to fill the gaps between the adjacent surfaces. This phenomenon is the result of capillary action. If the metal surfaces are not clean, the solder cannot wet the surfaces to be soldered, and the solder will not fill the joint. When a properly fluxed PCB having a plated through-hole is passed over a solder wave, the hole fills with solder and produces a fillet on the upper surface (component side) of the circuit board as illustrated in FIG. 2. It is not the pressure of the solder wave that forces solder up into the holes, but rather capillary action.
FIG. 1 depicts a typical wave soldering process for printed circuit boards. Molten solder is pumped up and over support plates to form a wave. A printed circuit board carried on a conveyor is passed over the solder wave at an angle such that the solder side (conductor side) of the board contacts the leading edge of the solder wave. Component leads are soldered to the solder pads on the board and plated through-holes such as via holes fill with solder.
It is not, however, always desirable to fill all holes in a PCB during the wave soldering process. Often PCB's are equipped with mounting holes and/or chassis ground holes which need to remain unobstructed in order to permit the subsequent insertion of mounting hardware such as bolts or screws or the attachment of grounding connectors. Most commonly, mounting holes and chassis ground holes on a PCB are one in the same. These holes are usually relatively large compared to VST's, PST's, and via holes. In order to provide electrical grounding contact to both sides of the PCB, such holes are usually plated through-holes.
FIG. 3 illustrates the common fate of such chassis ground mounting holes when a PCB is wave soldered. Solder partially (or even completely) occludes the hole and, by virtue of its being a plated through-hole, wicks up into the hole.
FIG. 4 illustrates the steps involved in mounting a PCB to a chassis by means of a bolt designed to be passed through a plated through-hole when such hole has become partially or completely occluded with solder. Solder must be reamed from the hole before the mounting hardware can be inserted. Grounding electrical contact is established from the chassis to the circuit board when the mounting bolt is tightened.
In the past it has been necessary to subsequently remove the solder which obstructs such holes by using a reamer (as shown in FIG. 4) or to reheat the solder and remove it in the molten state using a solder sucker or wick. Alternatively, one could apply a removable solder mask to the hole prior to wave soldering. One such removable solder mask is No. 2680 Aqua Strip Mask produced by Kenco Industries, Inc.
There are numerous disadvantages to the above-mentioned methods. All are labor-intensive and usually necessitate the addition of manual labor in an otherwise highly automated process. The reaming method is generally not effective in removing solder from the solder pad surrounding the mounting hole and thus the surface which contacts the head of the mounting bolt or contacts the stud or spacer on the chassis which is used to support the PCB is frequently uneven and thus an uneven strain can be placed on the board when the mounting bolt is tightened. This often results in delamination of the PCB in the region of the mounting hole.
The use of a solder mask also poses problems. The mounting holes of PCB's are often located at or near the periphery of the board. Most commonly, automated equipment for inserting components, wave soldering, and conveying PCB's throughout a manufacturing facility is designed to grasp or otherwise contact the circuit board on its edges. Thus, such equipment often becomes fouled with the solder mask material inasmuch as the solder mask must usually be applied right up to the edge of the board.
The present invention eliminates these problems.